1. Technical Field
The present invention relates to an electrostatic ultrasonic transducer, a method of manufacturing an electrostatic ultrasonic transducer, an ultrasonic speaker, a method of reproducing a sound signal, an super-directivity sound system, and a display device that are capable of improving an output sound pressure by increasing an effective membrane displacement of a vibrating membrane and increasing an opening ratio of radiating holes (through-holes) radiating a sound wave.
2. Related Art
An ultrasonic transducer outputs a modulated wave obtained by modulating a carrier wave in an ultrasonic wave band with a sound signal in an audible band, and reproduces a sound having sharp directivity.
FIGS. 15A and 15B are diagrams illustrating an example of a structure of an ultrasonic transducer according to the related art. Most of ultrasonic transducers according to the relate art are resonance-type ultrasonic transducers using piezoelectric ceramic. A piezoelectric ultrasonic transducer shown in FIG. 15A performs a conversion from an electrical signal to an ultrasonic wave and a conversion from the ultrasonic wave to the electrical signal (transmission and reception of an ultrasonic wave) by using piezoelectric ceramic as a vibration element.
A bimorph-type ultrasonic transducer shown in FIG. 15A includes two sheets of piezoelectric ceramics 61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67. The piezoelectric ceramics 61 and 62 are bonded to each other, and the leads 65 and 66 are connected to the surfaces of the piezoelectric ceramics 61 and 62 opposite to the bonding surface between the piezoelectric ceramics 61 and 62.
Since the piezoelectric ultrasonic transducer utilizes a resonance phenomenon of the piezoelectric ceramics, ultrasonic wave transmitting and receiving characteristics become superior in a relatively narrow frequency band near a resonance frequency thereof. However, since the piezoelectric transducer utilizes a sharp resonance characteristic of an element, a high sound pressure is obtained, but a frequency band is extraordinarily narrow. For this reason, a reproducible frequency band is narrow in an ultrasonic speaker that uses the piezoelectric transducer, and a reproducing sound quality becomes deteriorated, as compared with a loud speaker.
Different from the above-described piezoelectric transducer, the electrostatic ultrasonic transducer has been generally known as a wide range oscillation type ultrasonic transducer that can reproduce a high sound pressure over a high frequency band. FIG. 15B shows an example of a structure of a wide band oscillation type electrostatic ultrasonic transducer. The electrostatic ultrasonic transducer is referred to as a pull type, because a vibrating membrane only moves in a direction toward a electrode side.
The electrostatic ultrasonic transducer that is shown in FIG. 15B uses as a vibrator (vibrating membrane), a dielectric 131 (insulator), such as a PET (polyethyleneterephthalate) resin, which has a thickness in a range of about 3 to 10 μm. In regards to the dielectric 131, an upper electrode 132 formed by using a metal foil such as aluminum is integrally formed on a top surface of the dielectric 131 by means of a deposition process or the like, and a lower electrode 133 formed of brass is provided to come into contact with the bottom surface of the dielectric 131. A lead 152 is connected to the lower electrode 133, and the lower electrode 133 is fixed on a base plate 135 that is made of Bakelite.
Further, the upper electrode 132 is connected to a lead 153, and the lead 153 is connected to a direct current bias power supply 150. By means of the direct current bias power supply 150, the upper electrode 132 is always applied with a direct current bias voltage for upper electrode absorption in a range of about 50 to 150 V, and the upper electrode 132 is attracted to the side of the lower electrode 133. Reference numeral 151 indicates a signal source.
The dielectric 131, the upper electrode 132, and the base plate 135 are caulked by a case 130 together with metal rings 136, 137, and 138, and a mesh 139.
A plurality of minute grooves (unevenness portions), which have a size in a range of about several tens to several hundreds of micrometers and an irregular shape, are formed on a surface of the lower electrode 133 at the dielectric 131 side. Since the minute grooves form gaps between the lower electrode 133 and the dielectric 131, the distribution of the capacitance between the upper electrode 132 and the lower electrode 133 is minutely varied. The random minute grooves are formed by manually polishing the surface of the lower electrode 133. In the electrostatic ultrasonic transducer, a plurality of capacitors where sizes or depths of gaps are different are formed, which achieves a wide band of a frequency characteristic (JP-A-2000-50387 and JP-A-2000-50392).
As described above, the electrostatic ultrasonic transducer that is shown in FIG. 15B has been generally known as a wide band ultrasonic transducer (pull type) that can generate a relatively high sound pressure over a wide frequency band.
However, a maximum output sound pressure of the electrostatic ultrasonic transducer is slightly low, it is difficult to obtain an ultrasonic sound pressure necessary when obtaining a parametric array effect, and a ceramic piezoelectric element such as PZT or a high molecular piezoelectric element such as PVDF is used as an ultrasonic generator. However, the piezoelectric element has a sharp resonance point without depending on a type of material thereof, is driven with a resonance frequency, and is put to practical use as an ultrasonic speaker. Therefore, a frequency region capable of ensuring a high sound pressure is very narrow. That is, it has a frequency region of a narrow band.
In order to solve this problem, the applications have suggested the static ultrasonic transducer, as shown in FIGS. 16A and 16B (Japanese Patent Application No. 2004-173946). This structure is generally referred to a push-pull type, and is capable of simultaneously satisfying a wide band and a high sound pressure, as compared with the pull-type electrostatic ultrasonic transducer.
FIGS. 16A and 16B are diagrams illustrating an example of a structure of a push-pull type electrostatic ultrasonic transducer. Specifically, FIG. 16A is a diagram illustrating a sectional structure of a push-pull type electrostatic ultrasonic transducer, and FIG. 16B is a plan view illustrating a electrode when viewed from a vibrating membrane side. FIG. 16A is a cross-sectional view taken along the line X-Y of FIG. 16B.
In FIGS. 16A and 16B, the push-pull type electrostatic ultrasonic transducer includes a pair of electrodes 20 and 21 each having a conductive member formed of a conductive material functioning as an electrode, a vibrating membrane 22 that is interposed between the pair of electrodes 20 and 21 and has a conductive layer (vibrating membrane electrode) 221, and a supporting member 25 that holds the pair of electrodes 20 and 21 and the vibrating membrane 22.
The vibrating membrane 22 has insulating layers 220, and a conductive layer 221 that is formed of a conductive material, and the conductive layer 221 is applied with a direct current bias voltage having a single polarity (both a positive polarity voltage and a negative polarity voltage are possible) by means of a direct current bias power supply 27.
Further, the pair of electrodes 20 and 21 have the same number and a plurality of through-holes 24 at locations facing each other with the vibrating membrane 22 interposed therebetween. Between the conductive members of the pair of electrodes 20 and 21, an alternating current signal is applied by means of the signal sources 28A and 28B. Between the electrode 20 and the conductive layer 221, and between the electrode 21 and the conductive 221, capacitors are respectively formed.
According to this configuration, in the electrostatic ultrasonic transducer, the conductive layer 221 of the vibrating membrane 22 is applied with the direct current bias voltage having a single polarity (in this example, positive polarity) by mans of the direct current bias power supply 27. Meanwhile, the pair of electrodes 20 and 21 are applied with the alternating current signal by mans of the signal sources 28A and 28B. As a result, since the positive voltage is applied to the electrode 20 during a positive half cycle of the alternating current signal output from the signal sources 28A and 28B, an electrostatic repulsive force acts in a surface portion 23A of the vibrating membrane 22 that is not interposed between the electrodes of the vibrating membrane 22, and the surface portion 23A extends downward in FIG. 16A. At this time, since the negative voltage is applied to the electrode 21 that faces the electrode 20, an electrostatic absorption force acts in a rear surface portion 23B at the rear surface side of the vibrating membrane 22, and the rear surface portion 23B extends downward in FIG. 16.
Accordingly, a membrane portion of the vibrating membrane 22 that is not interposed between the pair of electrodes 20 and 21 is applied with an electrostatic repulsive force and electrostatic repulsion in the same direction. In the same manner with respect to the negative half cycle of the alternating current signal that is output from the signal sources 28A and 28B, in FIG. 16A, the electrostatic absorption force acts upward in the surface portion 23A of the vibration membrane 22, and the electrostatic repulsive force acts upward in the rear surface portion 23B in FIG. 16A. A membrane portion of the vibrating membrane 22 that is not interposed between the pair of electrodes 20 and 21 is applied with an electrostatic repulsive force and electrostatic repulsion in the same direction. In this way, while the vibrating membrane 22 is applied with the electrostatic repulsive force and the electrostatic repulsion in the same direction as the polarity of the alternating current signal is varied, a direction where the electrostatic force alternately acts is varied. Therefore, it is possible to generate a sound signal having a sufficient sound pressure level that is necessary when obtaining a strong membrane vibration, that is, the parametric array effect.
As such, the ultrasonic transducer that is shown in FIGS. 16A and 16B is referred to as a push-pull type because the vibrating membrane 22 receives a force from the pair of electrodes 20 and 21 and vibrates. The push-pull type electrostatic ultrasonic transducer has a capability that is capable of simultaneously achieving a wide band and a high sound pressure, as compared with the pull type electrostatic ultrasonic transducer where the electrostatic absorption force is only applied to the vibrating membrane.
As described above, in the push-pull type electrostatic ultrasonic transducer, a high direct current bias voltage is applied to the vibrating membrane and the alternating current voltage is applied to the electrodes, and thus the membrane portion vibrates due to an electrostatic force (attraction or repulsion) that is applied to the electrode and the vibrating membrane. In this case, in order to achieve the vibration in the ultrasonic wave band, the diameter of the hole of the vibrating portion needs to be several mm or less. For example, as shown in FIG. 17, it is required to form a transducer having a high following characteristic and a high output characteristic by providing the plurality of through-holes (vibration holes) 24 on the electrode 20.
FIGS. 18A to 18C are diagrams illustrating a structure of a electrode that is used in a push-pull type electrostatic ultrasonic transducer shown in FIGS. 16A and 16B and a process for manufacturing the electrode.
As described above, the electrode needs to be provided with the through-holes for radiating the sound wave, and through-holes of 1000 or more may be formed. The mechanical processing is suitable in terms of processing precision, but since instead of the mechanical processing, the etching is used because of the problem of the cost. However, there is a restriction between the diameter of the through-hole formed by the etching and the thickness. For example, it is difficult to manufacture with the etching process, the electrodes that have the through-hole diameter of 0.75 mm and the thickness of 1.5 mm and satisfy the predetermined processing precision.
Accordingly, as shown in FIG. 18A, a mask member 201 for forming the predetermined through-holes 203 is coated on a conductor (it is generally metal, and copper or stainless can be used as the conductor) 202 that has the thickness sufficiently smaller than the diameter of the through-hole, for example, the thickness of 0.25 mm, which is then subjected to the etching process. In this way, the plurality of conductors 202 are prepared in which the through-holes 203 are formed.
In addition, as shown in FIG. 18B, when the total thickness of the conductors is 1.5 mm, six sheets of conductors 202 are laminated in a state where all of the through-holes 203 are aligned. As shown in FIG. 18C, in a state where the laminated conductors 202 are pressed from both sides, the laminated conductors 202 are subjected to a thermal compressing process or a dispersion bonding process. As a result, it is possible to form an integral (metal-coupled) electrode having the thickness of 1.5 mm. FIGS. 18A and 18B show an example where square electrodes are manufactured. However, when the circular electrodes are manufactured, the circular conductor 202 is used.
Meanwhile, as described above, a plurality of through-holes for radiating the sound wave need to be formed in the electrodes of the electrostatic ultrasonic transducer. In this case, as shown in FIG. 16B, around the through-holes 24, the counter electrode portions 26 are disposed to make the electrostatic force applied to the vibrating membrane, and the electrostatic force is applied between the counter electrode portion 26 and the vibration region of the vibrating membrane 22 (portion of the vibrating membrane that is not interposed between the electrodes).
In this case, the diameter D1 of the through-hole 24 is set to half the diameter D2 of the electrode that forms the counter electrode portion 26. This relationship is set such that the relationship between the radiating efficiency of the sound wave and the membrane vibration amplitude becomes most excellent. For example, if the diameter of the through-hole becomes smaller (that is, if the area of the counter electrode portion 26 becomes larger), the electrostatic force becomes stronger, which increases the membrane vibration amplitude. However, the radiating area of the sound wave is decreased, which lowers the radiating sound pressure. Meanwhile, if the diameter of the through-hole becomes larger (that is, if the area of the counter electrode portion 26 becomes smaller), the radiating area of the sound wave is increased. However, since the electrostatic force becomes weaker, the membrane vibration amplitude is decreased, which lowers the radiating sound pressure.
The transducer is constructed according to the above-described relationships. However, in the structure according to the related art shown in FIG. 16B, the electrostatic force that is applied to the vibrating membrane is only applied to the outer circumferential portion of the vibration region, and it is difficult to generate the membrane vibration with high efficiency.
As described above, in the push-pull type electrostatic ultrasonic transducer according to the related art, the electrostatic force that is applied to the vibrating membrane is only applied to the outer circumferential portion of the vibration region, and it is difficult to generate the membrane vibration with high efficiency.